1. Field of the Invention
The present invention relates to a Multi-User Multiple Input Multiple Output (MU-MIMO) communication, and more particularly, to a method for transmitting pilot allocation information to a user and to a method of receiving such pilot allocation information at a user equipment.
2. Discussion of the Related Art
Future-generation mobile communication and wireless transmission systems are expected to employ a Multiple Input Multiple Output (MIMO) communication that uses a plurality of antennas for data transmission. The goal of future generation systems is to increase data rate and system capacity.
In a Single User Multiple Input Multiple Output (SU-MIMO) communication, a channel matrix H is formed between a Mobile Station (MS) with two or more antennas and a Base Station (BS) with two or more antennas. By contrast, in a MU-MIMO communication, a channel matrix H is formed between two or more MSs each having one or more antennas and a single BS with a plurality of antennas. The concept of uplink MU-MIMO is illustrated in FIG. 1.
Compared to first-generation MIMO communication systems based on the premise that a plurality of pieces of information are transmitted through a plurality of antennas between a BS and a single user, MU-MIMO communication forms a Multiple Input Multiple Output Spatial Multiplexing (MIMO SM) matrix between a BS and a plurality of users, as illustrated in FIG. 1 and FIG. 2. Derived from basic MIMO concepts, the MIMO SM channel matrix is improved because the values of its elements are less correlated. Therefore, if multiple users are considered at a given time, an improved channel matrix may be achieved because fewer correlations are expected among the antennas of the users.
As illustrated in FIG. 2, in SU-MIMO, the BS determines with which user it will form a 2×2 channel matrix at a given time to achieve the highest data rate. For example, a MIMO operation with a first user, User-1, may achieve a total data rate of 2.5 Mbps (=2.0+0.5), a MIMO operation with a second user, User-2, may achieve a total data rate of 2.0 Mbps (=1.5+0.5), and a MIMO operation with a third user, User-3, may achieve a total data rate of 1.8 Mbps (=0.8+1.0). Accordingly, the BS operates with User-1 in SU-MIMO as User-1 achieves the highest total data rate.
By contrast, in MU-MIMO, the data rate is determined according to signals transmitted to different users through a plurality of transmission antennas. For example, if the BS transmits signals to User-1 and User-2, a total data rate of 3.5 Mbps (=2.0+1.5) may be achieved. However, even in a MU-MIMO system, if the data rate between the BS and a single user at a given time is the highest data rate that may be achieved from among all possible user combinations, a MIMO operation may be performed between the BS and the single user. In this context it may be said that MU-MIMO encompasses SU-MIMO.
A brief description will be made of a channel estimation method and a pilot signal.
For detection of a synchronization signal, a receiver requires knowledge of a radio channel (i.e., attenuation, phase shift, time delay, etc.). Channel estimation is the process of estimating the amplitude and reference phase of a carrier. In a wireless channel environment experiencing irregular changes of a channel over time in the time and frequency domains, called “fading,” the amplitude and phase of the channel are estimated. This is channel estimation. In other words, channel estimation is equivalent to estimation of the frequency response of a radio interface or a radio channel.
For channel estimation, a reference value may be estimated using several pilot symbols from the BS by means of a two-dimensional channel estimator. As the pilot symbols are intended to help carrier phase synchronization and acquisition of BS information, they are transmitted at a high power level without carrying actual data. The transmitter and the receiver may carry out channel estimation using such pilot symbols. Specifically, the pilot symbol-based channel estimation is to estimate a channel using pilot symbols known to both the transmitter and the receiver and recover data based on the channel estimate.
Accordingly, the BS should notify each MS of pilot allocation information specifying pilots allocated to the MS in the MU-MIMO system. To this end, the MU-MIMO system needs additional signaling.